Casting cores and methods for making

ABSTRACT

Embodiments of the present invention include methods for forming ceramic articles, such as cores and core dies used in investment casting. The method may provide ceramic articles having improved strength and smoothness compared to articles manufactured by conventional methods. One embodiment is a method that comprises introducing a liquid comprising a silicon-bearing fluid into pores of a porous ceramic body; and heating the porous ceramic body in an oxygen-bearing atmosphere to form silica within the pores of the body.

BACKGROUND

This disclosure generally relates to investment casting, and more particularly, relates to materials for use in forming the ceramic cores employed in investment casting.

The manufacture of gas turbine components, such as turbine blades and nozzles, requires that the parts be manufactured with accurate dimensions having tight tolerances. Investment casting is a technique commonly employed for manufacturing these parts. The dimensional control of the casting is closely related to the dimensional control of a ceramic insert, known as the core (which is typically used to form internal surfaces, such as cooling passageways, within the casting), as well as the mold, also known as the shell (which typically corresponds to the external surfaces of the casting). In this respect, it is important to be able to manufacture the core and shell to dimensional precision corresponding to the dimensions of the desired metal casting, e.g., turbine blade, nozzle, and the like.

In addition to requiring dimensional precision in the formation of the ceramic core, the production of various turbine components requires that the core not only be dimensionally precise but also be sufficiently strong to maintain its shape and integrity during the firing, wax encapsulation, shelling, and metal casting processes. In addition, the core must be sufficiently compliant to prevent mechanical rupture of the casting during cooling and solidification. Further, the core materials generally must be able to withstand temperatures commonly employed for casting of superalloys that are used to manufacture the turbine components, e.g., temperatures generally in excess of 1,000° C. Finally, the core must be easily removed following the metal-casting process. The investment casting industry typically uses silica or silica-based ceramics due to their superior leachability in the presence of strong bases.

As explained in U.S. Pat. No. 6,494,250, ceramic cores for investment casting typically are formed by injection molding, transfer molding or pouring into a suitably shaped core die a ceramic-bearing mixture that includes, among other things, ceramic powder dispersed in a binder or carrier. After the so-called “green” core is removed from the die, it is fired in one or more steps to remove the carrier and strengthen the core via sintering for use in casting operations. As a result of removal of the carrier and other sacrificial additives, the fired ceramic core is generally porous.

In one particular example, investment casting cores can be made using low pressure injection molding techniques such as that described in U.S. Pat. No. 7,287,573. The process described therein generally includes dispersing a ceramic powder to form a slurry in a silicone fluid, wherein the silicone fluid includes silicone species having alkenyl and hydride functionalities. Once a stable suspension is formed, a metallic catalyst is added and the desired part is formed. Depending on the particular binder liquid and metallic catalyst employed, a heating step may then be applied to effect a catalyzed reaction among the siloxane species, thereby curing the formed suspension into a green body. The silicone species cross link in the mold, yielding a dispersion of ceramic particles in a rigid silicone-based polymeric matrix. The so-formed silicone polymeric matrix may be substantially decomposed to produce a silica char by further heating at a higher temperature.

In another example, additive manufacturing processes, such as three-dimensional printing, are applied to form the dies used to form cores (as in U.S. Pat. No. 7,413,001, for example), or directly to form the cores themselves from ceramic-slurry “inks” that are subsequently fired to form porous ceramic cores.

One of the challenges in ceramic core processing is that cores formed by these various processes can be fragile and susceptible to cracking during typical handling and/or transportation, or during the investment casting process itself Moreover, in many additive manufacturing processes, the surface of the resultant ceramic core is rougher than that obtained in the traditional injection molding process. It is often desirable to maintain a low surface roughness in the eventual cast cooling passages (for part life, or other considerations). Therefore, a need remains for processes for making ceramic cores that are strong with improved smoothness.

BRIEF DESCRIPTION

Embodiments of the present invention are provided to meet this and other needs. One embodiment is a method that comprises introducing a liquid comprising a silicon-bearing fluid into pores of a porous ceramic body; and heating the porous ceramic body in an oxygen-bearing atmosphere to form silica within the pores of the body.

DETAILED DESCRIPTION

Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, and “substantially” is not to be limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged; such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise.

In the following specification and the claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. As used herein, the term “or” is not meant to be exclusive and refers to at least one of the referenced components being present and includes instances in which a combination of the referenced components may be present, unless the context clearly dictates otherwise.

As used herein, the terms “may” and “may be” indicate a possibility of an occurrence within a set of circumstances; a possession of a specified property, characteristic or function; and/or qualify another verb by expressing one or more of an ability, capability, or possibility associated with the qualified verb. Accordingly, usage of “may” and “may be” indicates that a modified term is apparently appropriate, capable, or suitable for an indicated capacity, function, or usage, while taking into account that in some circumstances, the modified term may sometimes not be appropriate, capable, or suitable.

Embodiments of the present invention include methods for forming ceramic articles, such as cores and core dies used in investment casting. The method may provide ceramic articles having improved strength and smoothness compared to articles manufactured by conventional methods. Fabrication of cores, for instance, having higher strength, may allow thin or hollow parts to be fabricated at higher yield by preventing breakage that occurs during downstream processing. Moreover, the ability to produce smoother surfaces ultimately may reduce surface finishing costs and improve part life via more efficient cooling.

In one embodiment, a liquid is introduced into the pores of a porous ceramic body. Examples of ceramic materials that may be included in the ceramic body include oxide materials, such as silica, alumina, and various silicate materials, among others. In particular embodiments, the ceramic body includes silica, zirconium silicate, or a combination of silica and zirconium silicate. Such materials often see use in fabrication of cores for investment casting, for instance. The introduction is done by contacting the surface of the body with the liquid, as, for instance, by dipping the body into a quantity of the liquid. Infiltration of the pores by the liquid is driven at least in part by capillary action, although in some embodiments vacuum infiltration is performed to provide further driving force for incursion of the liquid into the porous network. Vacuum infiltration may be followed with a step of exposing the infiltrated body to a pressure at or exceeding ambient pressure, to enhance the degree to which a driving force is provided to infiltrate the liquid into the porous body.

The liquid includes a silicon-bearing fluid, such as a siloxane or a silane, or any other silicon-bearing compound that may serve as a precursor to a silicon-bearing ceramic material in a process such as described herein. Non-limiting examples of suitable silicon-bearing fluids include alkenyl siloxanes of the general formula (I):

wherein R1, R2, and R3 each independently comprise hydrogen or a monovalent hydrocarbon, halocarbon, or halogenated hydrocarbon radical; X a divalent hydrocarbon radical; and a is a whole number having a value between 0 and 8, inclusive. The terms “monovalent hydrocarbon radical” and “divalent hydrocarbon radical” as used herein are intended to designate straight chain alkyl, branched alkyl, aryl, aralkyl, cycloalkyl, and bicycloalkyl radicals. A specific example of such a material is 1,3,5,7-tetravinyl 1,3,5,7-tetramethylcyclotetrasiloxane (United Chemical Technologies, product number T2160). Further non-limiting examples of silicon-bearing fluids include hydrosiloxanes having hydrogen directly bonded to one or more of the silicon atoms, and therefore containing a reactive Si—H functional group. A particular example of such a material is a hydride-functional organosilicate resin such as the material available from Momentive Performance Materials under the trade name 88104EX.

The selection of any particular silicon-bearing fluid depends in part upon the desired flow properties of the liquid. For example, a relatively viscous liquid may prove difficult to achieve satisfactory infiltration into the ceramic body. On the other hand, liquids having relatively low viscosity tend to also have relatively high vapor pressure, and if the silicon-bearing fluid has an unduly high vapor pressure, it will tend to volatilize during further processing before conversion to silica or other ceramic product. In one embodiment, the liquid is neat, meaning it consists essentially of a single species of silicon-bearing fluid. In alternative embodiments, the liquid includes a mixture of fluid components, wherein, for instance, in addition to the silicon-bearing fluid there is added a second fluid, which may or may not also include silicon. In a particular embodiment, the second fluid has a lower viscosity than the silicon-bearing fluid, which reduces the overall viscosity of the resultant liquid, which may improve the ability of the liquid to infiltrate the ceramic body. An example of such a mixture is a liquid that includes the aforementioned 88104EX as the silicon-bearing fluid and 1,3,5,7-tetravinyl tetramethyl cyclotetrasiloxane (also known in the art as “D4Vi” and “D4 Vinyl”) as the lower viscosity second fluid. The proportions of the mixture are selected to provide a suitably concentrated source of silicon within the pores while also providing a viscosity that allows acceptable penetration into the porous network of the ceramic body.

In some embodiments, the viscosity of the liquid is in a range up to about 1,000 centistokes, up to about 300 centistokes where a lower viscosity is desirable, and up to about 100 centistokes in particular embodiments where even lower viscosity is desirable (such as where the pores are fine, particularly tortuous, and/or where deep and rapid infiltration is desired). In one illustrative embodiment, the liquid has a viscosity in a range from about 5 centistokes to about 30 centistokes.

In embodiments in which the liquid includes both a silicon-bearing fluid and a second, lower viscosity fluid, the method optionally further includes volatilizing the second fluid after the liquid has been introduced into the pores of the ceramic body. This may be achieved, for example, by heating the infiltrated body to a suitable temperature at which the second fluid volatilizes at an acceptable rate while leaving the silicon-bearing fluid to remain in the pores of the body. This heating step may be done separately from the reacting step, or it may be coincidental with the reacting step, wherein during the heat-up to a reacting temperature, the second fluid volatilizes as the temperature increases. In some embodiments, the volatilizing step includes heating the infiltrated ceramic body to a first temperature that is above ambient but below the reacting step temperature, and holding at this first temperature for a desired time (typically a length of time sufficient to volatilize substantially all of the second fluid).

Depending on the selection of first and second fluids, in some embodiments of the present invention the liquid comprises a mixture of silicone monomers. This mixture may be processed as noted above, volatilizing the component of the mixture having the lower vapor pressure, without ever performing a step to cure the mixture, that is, cause the components of the mixture to react, to form a cross-linked silicone polymer. However, in alternative embodiments, the method further comprises curing the mixture of silicone monomers. Various methods for curing silicone monomers are widely known in the art, and the selection of which method to use in the presently described embodiments will of course depend in part on the selection of monomers in the liquid. Curing the monomers involves performing a crosslinking reaction, typically involving one of three basic reaction types: peroxide cure, which is a free-radical reaction catalyzed by a peroxide and activated by heat; condensation cure, which is catalyzed by a tin-bearing or titanium-bearing compound, and which can be activated by heat or, in some systems, by moisture (i.e., the so-called Room Temperature Vulcanized, or RTV systems); and addition cure, catalyzed by a complex of a platinum-group metal (often platinum or rhodium) and activated by exposure to ultraviolet light, or by heat. As thermally cured systems are generally convenient for use in the method described herein, in specific embodiments the liquid further comprises a catalyst suitable for supporting one of the thermally activated reaction types noted above, and thus the curing step includes thermally curing the mixture of silicone monomers in the presence of the catalyst.

In one general, non-limiting example involving an addition-cure reaction, the silicon-bearing fluid includes an alkenyl siloxane of formula (I), above, and a hydrosiloxane containing a reactive Si-H functional group. Specific examples of suitable alkenyl siloxanes and hydrosiloxanes for use in such embodiments of the present invention are described in, for instance, U.S. Pat. No. 7,287,573. A metallic catalyst compound is added to the liquid, and cross-linking of the silicone monomers may be accomplished by utilizing a metal catalyzed reaction of the silicone alkenyl groups and the silicon bonded hydrogen groups. The metal catalyst, such as a platinum-group metal catalyst, can be selected from such catalysts that are conventional and well known in the art. Suitable metallic catalysts include, but are not intended to be limited to, the Pt divinylsiloxane complexes as described by Karstedt in U.S. Pat. No. 3,715,334 and U.S. Pat. No. 3,775,452; Pt-octyl alcohol reaction products as taught by Lamoreaux in U.S. Pat. No. 3,220,972; the Pt-vinylcyclosiloxane compounds taught by Modic in U.S. Pat. No. 3,516,946; and Ashby's Pt-olefin complexes found in U.S. Pat. Nos. 4,288,345 and 4,421,903.

A typical thermal cure involves heating the body to a temperature greater than about room temperature, often up to about 120° C., such as a temperature in a range from about 50° C. to about 100° C. The time necessary to cure the reactants is dependent on the particular monomers, solvent, temperature, and metallic catalyst compound. Generally, the ceramic body containing the silicon-bearing fluid is preferably heated at an elevated temperature (i.e., greater than room temperature) for at least about five minutes, such as a period of about 5 to about 30 minutes in order to polymerize and crosslink the monomers.

In some embodiments, the liquid is a slurry in which a plurality of particles is suspended within the silicone-bearing fluid. The particles are typically a ceramic material, such as an oxide. Particular examples of suitable oxides include, but are not limited to, yttria, zirconia, fumed silica, alumina, and magnesia. Combinations of different particulate materials are also suitable. Addition of these fine particles to the fluid may allow for enhanced deposition of solid oxide material within the pores, providing enhanced strengthening and/or smoothing of the surface of the ceramic body. Selection of the concentration and size of particles depends in part upon the size of the pores in the porous body and the desired flow characteristics of the resulting slurry. Typically the selection is made to balance the desire for efficient strengthening or smoothing with the desire for efficient infiltration of the liquid into the pore network of the body.

After the liquid has been introduced into the pores of the porous ceramic body, with or without the curing step described above, the porous ceramic body, now with a silicon-bearing material disposed within its pores, is heated in an oxygen-bearing atmosphere, thereby causing the silicon-bearing material to react with the oxygen to form silica within the pores of the body. This heating step typically involves increasing the temperature of the ceramic body to a temperature sufficient to drive the conversion of the material resident within the pores to silica in a desired length of time. For instance, in some embodiments the ceramic body is heated to a temperature greater than about 200 degrees Celsius, and then held at this temperature for sufficient time to allow substantially all of the convertible silicon-bearing material resident within the porous network of the body to convert into silica. “Convertible” silicon-bearing material in this regard means silicon-bearing material that is present in a form amenable to reaction with oxygen under the processing conditions; typically this will include siloxane, silane, silicone, and like materials that are prone to reaction with oxygen under elevated, oxidizing conditions, but will exclude materials such as silica, silicon carbide, and other materials having a higher stability at high temperatures. In particular embodiments, the heating step includes heating the body to a temperature greater than about 400 degrees Celsius; a higher temperature generally provides faster reaction times.

Those skilled in the art will recognize that the rate at which the body's temperature is increased during the heating step may be considered carefully, particularly where the silicon-bearing material in the pores of the ceramic body is still in liquid form or is otherwise prone to outgassing during heating. If vapor is produced so rapidly as to unduly build up pressure within the pores, damage may be done to the ceramic body. Thus in some embodiments, heating is performed gradually, either through a sufficiently slow but continuous increase, or by heating the body through one or more intermediate temperatures, holding the body at each intermediate temperature for intervals of time to allow for controlled generation of vapor. The design of such “heating profiles” is done taking into account, among other things, the nature of the material deposited in the pores of the body, the extent of the porous network, and the desired rate of processing. In some embodiments, after a first heating stage to a temperature used to form the silica within the pores, a second, higher temperature heat treatment is performed to sinter the material in the pores, thereby further strengthening the material and enhancing its ability to reinforce the ceramic body. The sintering step is typically performed at temperatures above about 900 degrees Celsius, with 1000 degrees Celsius as a non-limiting example.

As noted above, in some embodiments of the method described herein, the porous ceramic body includes, or is in its entirety, a core of the type used for investment casting. Cores formed in accordance with U.S. Pat. No. 7,287,573 referenced above, for example, may undergo a first, lower temperature firing step, and then a later sintering step at significantly higher temperature. The introduction of the liquid, followed by other steps in accordance with embodiments described herein, may be desirably performed after the first firing step but before the sintering step. Thus when the sintering step is performed in these embodiments, both the ceramic body and the material resident within the pores may be sintered simultaneously.

EXAMPLES

The following examples are presented to further illustrate non-limiting embodiments of the present invention.

A porous ceramic test bar was vacuum infiltrated with a liquid comprising about 45 volume % low viscosity liquid silicone resin (Momentive 88104) and about 55 volume % octamethylcyclotetrasiloxane (D4). The infiltrated bar was dried overnight in a drying oven at 80 degrees Celsius, and then fired in air at 940 degrees Celsius to convert the silicone to silica. The treated bar and an untreated control bar were subjected to modulus of rupture testing at ambient temperature. The treated bar showed a 49% increase in modulus of rupture compared to the untreated control bar.

While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. 

1. A method comprising: introducing a liquid comprising a silicon-bearing fluid into pores of a porous ceramic body; and heating the porous ceramic body in an oxygen-bearing atmosphere to form silica within the pores of the body.
 2. The method of claim 1, wherein the fluid comprises a siloxane or a silane.
 3. The method of claim 1, wherein the liquid is neat.
 4. The method of claim 1, wherein the liquid further comprises a second fluid.
 5. The method of claim 4, wherein the second fluid has a lower viscosity than the silicon-bearing fluid.
 6. The method of claim 4, further comprising volatilizing the second fluid after the liquid has been introduced into the pores of the ceramic body.
 7. The method of claim 1, wherein reacting comprises heating the fluid to a temperature above about 200 degrees Celsius.
 8. The method of claim 1, wherein the liquid comprises a mixture of silicone monomers.
 9. The method of claim 8, further comprising curing the mixture of silicone monomers prior to the reacting step.
 10. The method of claim 9, wherein the liquid further comprises a catalyst, and wherein curing comprises thermally curing the mixture of silicone monomers in the presence of the catalyst.
 11. The method of claim 1, wherein the liquid further comprises a plurality of particles suspended within the silicon-bearing fluid.
 12. The method of claim 11, wherein the plurality of particles comprises oxide particles.
 13. The method of claim 1, wherein introducing comprises performing vacuum infiltration of the liquid into the pores of the body.
 14. The method of claim 1, wherein the porous ceramic body comprises a core for investment casting.
 15. The method of claim 1, wherein the ceramic body comprises silica, zirconium silicate, or a combination of zirconium silicate and silica. 